UserForm, Vlookup Error when the form is done.

I have almost completed my form (I even did some without help.
It is working correctly, my userform pulls from the right lists, fills in necessary info and adds it to the right place in the form - But when I click the CommandButton it inserts everything in the form at the right place and immediately returns this error: Unable to get the Vlookup Property of the WorsheetFunction Class, and the Debug highlights the first row of my Vlookupsrow:

Private Sub CboPart_Change()

Me.TxtMkt = WorksheetFunction.VLookup(Me.CboPart, Sheets("Parts").Range(CboMainframe.Text), 2, False)
Me.TxtPartNo = WorksheetFunction.VLookup(Me.CboPart, Sheets("Parts").Range(CboMainframe.Text), 3, False)

I suspect that it is because I clear the ComboBox "Part" in my CommandButton code - but I need the box to clear.
Should I make the Vlookups a Command Button Function?

I am attaching file